1. Field of the Invention
The present invention relates in general to RF power amplifiers and RF amplification methods. The present invention is also more specifically related to DC supply (bias) feed networks for RF power amplifiers.
2. Description of the Prior Art and Related Information
Radio frequency (RF) power amplifiers are commonly used in numerous applications, such as base stations used in wireless communication systems. The signals which may be amplified by an RF power amplifier include an RF high frequency modulated carrier, for example having a fundamental frequency of around 2 gigahertz (GHz). The base band signal component which modulates the carrier is commonly at lower frequency Depending on the application, the base band (video) bandwidth can be anything from 30 kilohertz (KHz), such as for older cellular telephones, to 20 megahertz (MHz) or more, such as for multi-carrier W-CDMA (Wideband-Code Division Multiple Access) applications such as UMTS (Universal Mobile Telecommunication System).
The two primary goals of RF power amplifier design are linearity over the range of power operation and efficiency. Linearity is simply the ability to amplify without distortion while efficiency is the ability to convert DC to RF energy with minimal wasted power and heat generation. Both these requirements are critical for modern wireless communication systems but mutually exclusive in nature. This is due primarily to the bandwidth requirements of modern wireless communication systems which are placing increasing demands on amplifier linearity.
One source of distortion in RF power amplifiers which is increasingly significant at higher video bandwidths is related to the DC power supply circuit for supplying power to the active devices, e.g., LDMOS transistors, employed in the amplifier. The finite impedance in the amplifier DC supply circuit results in a voltage drop at the transistor output terminal, and hence a parasitic modulation of the drain voltage. This parasitic modulation interacts with the RF signal and creates intermodulation distortion products (IMDs), resulting in an increase in the amount and complexity of distortion present at the amplifier output. Ideally, to minimize this distortion, the bias circuit should have a low impedance at the frequency of the signal modulation, i.e., a low video impedance. The move towards increasing signal bandwidth (e.g. 20 MHz for 4-carrier UMTS) means that the frequency range over which low video impedance is required is also increasing. Due to inherent impedances in the bias circuit elements at MHz frequencies, it is becoming increasingly difficult to maintain low video impedance across the video bandwidth.
The problem of maintaining low impedance across the video bandwidth is made much more difficult by the second key requirement of RF power amplifiers; good efficiency. Good amplifier efficiency requires that there be minimal power losses due to impedance mismatching at the amplifier output. An output matching circuit is typically provided to match the impedance at the output to the device impedance and minimize power loss, whilst achieving the desired power bandwidth of operation. Ideally, the bias circuit should not impact on this output impedance matching. Therefore, at the RF carrier frequency, the bias circuit should have a high impedance, to prevent loading of the matching circuit. Therefore, there are two requirements which tend to conflict with each other. By keeping the bias circuit video impedance low, linearity is maintained and distortion of the video signal is minimized but at the same time, the bias circuit must have high impedance at the RF carrier frequency to avoid power loss.
Conventional approaches to this problem employ a number of decoupling capacitors in parallel to reduce video impedance. Each capacitor is chosen for its low impedance over a certain frequency range, this bandwidth being centered on the series self-resonance of the capacitor. An RF blocking inductance, such as a short-circuit quarter-wave transmission line at the RF carrier wavelength, is then provided between the decoupling capacitors and the transistor drain/collector terminal, to provide a high impedance to the RF signal, thereby minimizing the effect of the bias network on the RF output match. Such an approach is described, for example, in U.S. Pat. No. 6,081,160 to Custer et al. FIG. 1 shows such a prior bias circuit coupled between a DC voltage supply port 1 and a transistor drain or collector port 2. As shown, the decoupling capacitors 4, having capacitances C1–C4, are connected to the output 2 of the transistor via an inductor/quarter-wave transmission line 3. However, the inductance of the quarter-wave transmission line becomes increasingly significant as the video/modulation frequency increases. This inductance is in series with the decoupling capacitors and has the effect of significantly lowering the series resonance of the high video frequency decoupling capacitors as measured at the device output terminal 2. Therefore, at higher video frequencies this inductance can increase the video impedance causing distortion by parasitic modulation of the RF signal.
The significance of the problem of distortion due to bias circuit impedance is increased by its resistance to known techniques for eliminating distortion. In order to reduce the distortion of RF power amplifiers, several techniques have been employed, amongst which, feed forward linearization is most popular for its wideband performance. Predistortion linearization techniques, such as adaptive digital predistortion (ADPD) linearization, have also been employed for reducing IMD components in RF power amplifiers. However, predistortion cannot be effectively employed to reduce distortion caused by varying bias circuit impedance across the video bandwidth. The interaction of the bias supply impedance with the RF signal creates time-varying RF intermodulation distortion products, a phenomenon generally known as the memory effect. The memory effect is difficult to correct even by the most sophisticated predistortion linearization techniques used in power amplifier systems. The distortion caused by a variation in bias circuit video impedance is thus extremely difficult to remove in any practical predistortion linearization scheme. Therefore, the variation in bias circuit video impedance creates a barrier to increasing the effectiveness of predistortion linearization in wide bandwidth applications.
Although there have been attempts to address the above outlined problem, a fully satisfactory solution has not been provided. Accordingly, there exists a need to address the problem of bias circuit video impedance for wide bandwidth applications and the associated RF amplifier memory effects problem so as to improve the amplifier performance.